As I live in Australia I use ancestry.com.au. Depending on where you live, or the main collections you want to use, you might use ancestry.com (United States), ancestry.co.uk (United Kingdom), or ancestry.ca (Canada).
Ancestry has a number of record collections that come from a range of countries including United States, United Kingdom, Australia, Scotland, and Canada. Most collections require a subscription.
Ancestry also provides the option to add your own family tree, and if you use Family Tree Maker, the ability to synchronise your family tree in Family Tree Maker with a tree on ancestry. You can also search other public member trees. Other family trees should be used as a guide only, until you are able to verify the information against other sources, but they can provide a good starting point for finding more information.
As well as record collection, the site also provides resources to help you with your family history research. They also have a blog – the link to this blog can be found in my Genealogy Blogs section in the right sidebar.
Each week, I will be checking to see if there are any new or updated collections on the Ancestry site, and will add a post to my blog if there are any.